docs(contributing): update squash and merge link to canonical documentation URL

What does this MR do?

Updates the \Squash commits when merge request is accepted\ link in \CONTRIBUTING.md\ from the instance help path to the canonical documentation URL:

• \https://gitlab.com/help/user/project/merge_requests/squash_and_merge\ -> \https://docs.gitlab.com/user/project/merge_requests/squash_and_merge/\